1. Field of the Invention
The present invention relates to a positive electrode material for a lithium secondary battery, to a method for producing the same, and to a lithium secondary battery using the same. More particularly, it relates to a novel material obtained by improving a positive electrode material for a lithium secondary battery having a Li—Ni—Co—O or Li—Ni—Co—Ba—O system composition, to a method for producing the same, and to a lithium secondary battery using the novel material.
2. Description of the Related Art
Recent years have seen various improvements in a positive electrode material for a lithium secondary battery. As a positive electrode material for a high-capacity secondary battery, a material having a Li—Ni—Co—O or Li—Ni—Co—Ba—O system composition can be listed.
For example, there is a positive electrode material which is a compound represented by a chemical formula Li1-x-aAxNi1-Y-bByO where
A: one alkaline earth metal element of either strontium or barium, or at least two alkaline earth metal elements selected from magnesium, calcium, strontium and barium
B: at least one transition metal element other than Ni
X: a total number of moles of A and 0<X≦0.10
Y: a total number of moles of B and 0<Y≦0.30
a: −0.10≦a≦0.10
b: −0.15≦b≦0.15
(see, e.g., Japanese Unexamined Patent Publication No. HEI 9-17430 (pp. 2-8)).
There is another positive electrode material which is a compound represented by a chemical formula Li1-x-aAxNi1-Y-bBYO and forming secondary particles with an average diameter of 5.0 μm to 50 μm as an aggregate of primary particles with an average diameter of 0.01 μm to 5.0 μm where
A: strontium or barium
B: at least one transition metal element
X: a total number of moles of strontium or barium and 0<x≦0.10
Y: a total number of moles of all transition metal elements other than Ni and 0<Y≦0.30
a: −0.10≦a≦0.10
b: −0.15≦b≦0.15
(see, e.g., Japanese Unexamined Patent Publication No. HEI 10-79250 (pp. 2-7)).
Although the foregoing materials are excellent in cycle performance of a secondary battery when used in a positive electrode for a lithium secondary battery, no mention has been made to the thermal stability, capacity, rate performance, and charge/discharge efficiency thereof.
The present inventors had conducted research on a positive electrode material for a lithium secondary battery, made a further examination of an amount of Ba in Li—Ni—Co—Ba—O system technology similar to the technology mentioned above, and proposed a material giving a high thermal stability and a large capacity under a limited range of Ba content (see, e.g., Japanese Unexamined Patent Publication No. 2001-173285 (pp. 3-11)).
As a result of pursuing research on the improvement of the properties of the positive electrode material for a lithium secondary battery, the present inventors have developed a material with more excellent charge/discharge efficiency and cycle performance.